October 14, 2006

Plato is one of the most conspicuous craters on the Moon. It is a big (~95 km in diameter) dark spot, prominently placed on the northern edge of Mare Imbrium. Observers invariably check Plato when looking at the Moon because there have been many reports of unusual occurrences on its floor. Mists, flashes of light and changes in indistinct albedo spots and smudges have been reported, and a multitude of tiny craterlets have been claimed. The question is: Do any of these reports represent actual changes on the surface of the Moon, or are they all errors made by over-zealous observers? To evaluate some of the classic reports of change in Plato, I compare the best modern telescopic and spacecraft images with the early drawings. All of the drawings and telescopic images have been digitally scanned, contrast stretched, sharpened, rectified (so that Plato is circular in outline - matching spacecraft views) and enlarged to the same scale. For ease of reference the largest craters have been numbered on Lunar Orbiter IV, frame H127.

(Lunar Orbiter IV)

1824: Lohrman’s map shows no craters or light spots on Plato’s floor.

1837: Beer and Madler’s classic map of the Moon shows no distinctive craters on Plato’s floor (Greek letters for rim peaks are plotted on the floor). Three bright stripes 5-10 km wide cross the floor from south to north, and a perpendicular stripe connects two of them.

Beer and Madler

1878: Schmidt’s Charte der Gebirge der Mond (Chart of the Mountains of the Moon) is apparently the first map to show craters on Plato’s floor. His map clearly shows and locates craters 1,3 and 5, but crater 4 is mis-located, and the topmost of his craters either does not exist or is number 6, grossly misplaced. The map shows no albedo features.

Schmidt

1897: Molesworth mapped considerable detail in Plato during observations in 1896-7. His schematic map appears to correctly plot craters 1-9 from the Orbiter image (although the relative sizes are not always correct). He also indicated streaks by dashed lines.

Molesworth

1915: Hallowes’ drawing of Plato under high sun illumination portrays well the triangular patch of bright material on Plato’s southwest (top right) floor quadrant. The triangle at the bottom right does not correspond with floor markings seen on Clementine images (below) or high sun telescopic photos.

Hallowes

1947: Thornton published a sketch of an observation from 9 Oct. 1945 when he “saw at the Western rim, just inside the wall, a minute but brilliant flash of light….it resembled the flash of an A.A. [anti-aircraft] shell exploding in air at a distance of about ten miles. In colour it was on the orange side of yellow.” Interestingly, the Orbiter IV photo shows two small craters (red arrows) near the spot Thornton specified. Unfortunately, neither of these looks any fresher than many other similar craters on the floor of Plato.

Thornton (the arrow above marks the flash location)Lunar Orbiter IV-H127, full resolution view of the area where Thornton reported seeing a brilliant flash and speculated that it could be an impact formation of a new crater. Red arrows mark craters closest to supposed impact point (the black arrow and X point to a tiny rille - image from Schultz, 1976).

1931: Gooadacre’sThe Moon includes many craters all of about the same size. Clearly the relative sizes are simply ignored, and some of the craters are merely representative in their locations.

Goodacre

1955: Wilkins and Moore’s The Moon was the apex of the British school of selenology. Their book includes this drawing of Plato done by Wilkins on 3 April 1952 using 400X on the 33” Meudon refractor in Paris. It shows craters 1, 3, 4, 5 and 7. Some of the white spots are near craterlets seen on the Orbiter IV and Clementine, but others are not. The most amazing feature of this drawing is the bent shadow of the peak Gamma. Comparison with C3698, taken under similar lighting conditions, shows that the peak is not bent and reveals other discrepancies for the northern portion of the shadow.

Wilkins-Meudon

How many craters are on Plato’s floor - and do they move or change appearance? In 1892, Prof. W.H. Pickering of Harvard College Observatory used a 13” telescope in Arequipa, Peru to detect 71 craterlets and spots on the floor of Plato. Comparison of Pickering’s estimates of the diameters of the five largest craters with my measurements on the high resolution Orbiter IV frame, H 127 reveal that except for the largest crater, he underestimated crater sizes by 60 to 100 percent.

Diameters of Craters on the Floor of Plato

Crater

Pickering

Wood

1

2.2

2.5

2

1.3

2.1

3

1.3

2.0

4

0.9

1.9

5

0.9

1.7

ConclusionsIn general, reports of the appearance and disappearance of small craters (all at the limits of detectability) and changes of the light spots almost certainly are fictitious. Only Molesworth’s drawing accurately represents the numbers and locations, but his relative sizes are not always correct. Changes in solar elevation and seeing conditions, as well as observer skill, care and experience, probably account for most reports of changes. Plato is unchanging, observing conditions and observer skill are the variables!